powerpc/spufs: Remove double check for non-negative dentry

This patch removes an unnecessary double check if the dentry returned by
lookup_create() is actually non-negative. Since lookup_create() itself returns
an error in this case just remove the check.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>